Why do chemical reactions happen? | 16-18 years

In this activity, entropy is introduced to students in a qualitative way, as a means of explaining why chemical reactions happen.

The question ‘Why do chemical reactions happen?’ is a basic one to answer. The best time to approach this issue is within a series of lessons on thermodynamics, once students are used to calculating enthalpy change values.

Learning objectives

Students will know and understand that:

• Disorder increases when spontaneous chemical reactions occur.
• An increase in disorder can be represented by wider distribution of energy or particles.
• Thermodynamics is responsible for determining whether chemical reactions ‘go’ or not and that reactions themselves are not ‘alive’, so cannot ‘decide’.

Sequence of activities

Introduction

1. The issue of why chemical reactions happen can be used to introduce the session. List views about this from the students, with little discussion. Retain the list for review at the end.
2. Immediately after the discussion, give each student a copy of ‘Will the reaction go?’.

Will the reaction ‘go’?

Organise students into groups of three or four. Circulate and support as the groups:

1. Consider their responses to the statements.
2. Discuss the statements to reach agreement.
3. Agree an answer to the question.
4. Select a spokesperson to feedback to the whole group.

Plenary 1

In a plenary:

• Hear the responses from each group.
• Ask for explanations supporting their views. (Do not comment on the answers at this stage.)

After the discussion, introduce the next activity.

Demonstration

Direct the students to make careful observations during the demonstration. Carry out the demonstration ‘Two reactions’.

Help students to notice:

• Reaction 1 – gas, liquid and solid products are formed; temperature decreases.
• Reaction 2 – heat and light are produced; iodine gas may be formed; temperature increases.

Pose questions to develop their ideas from their observations:

• Were their predictions correct?
• How can one reaction be endothermic and the other exothermic?
• Why do both reactions ‘go’?

It’s all about chance

Introduce the next activity that looks at reactions from a different perspective. Give each student a copy of ‘It’s all about chance’, then support the groups as they:

• Work through the sheet.
• Consider chance as a driving force for events.
• Consider how chance applies to chemical reactions.
• Select a spokesperson to feedback to the class.

Plenary 2

In a plenary:

1. Invite responses from each group.
2. Introduce the point that odds predict the most likely event to occur, unless there is some special factor preventing it, eg sportsman taking drugs, cheating on the lottery, stopping smoking, a football team getting a better coach or players etc.
3. Explain that this is also true of chemical reactions – the ‘most likely’ event is the one that will occur, unless something prevents it.
4. Ask ‘How do we know what is the most likely event?’.
5. Explain that this is not to do with exothermic or endothermic, but must be something else.

Role play

1. With the whole group, orchestrate the role play ‘Give us some energy!’.
2. When completed, ask questions to draw some meaning from the role play:
   • What happened to the energy?
   • Could the energy be used again after the reaction?
   • If this were the environment, what would have happened to its temperature?
   • Which reaction does this help to explain?
3. Introduce the notion that the energy has become disordered as a result of the reaction.
4. Explain that the role play represents the aluminium/iodine reaction and ask what else can become disordered.
5. Send the students back to their groups to look again at the first reaction.
6. Tell them to try to work out what has become more disordered.

Plenary 3

In a closing plenary:

1. Invite students to explain what makes reactions ‘go’.
2. Check for understanding that energy and particles can become more disordered.
3. Introduce the term entropy as a measure of disorder.
4. Check that students realise entropy always increases when chemical reactions occur.
5. Ask what factors may prevent reactions happening. For example:
   • Rate of reaction may be slow.
   • Catalyst may be needed.
   • Pressure and / or temperature may be too low.
   • Intermolecular bonds may form preventing reactants mixing etc.
6. Review the list generated at the start of the lesson.

Before finishing

Ask students to write a short summary reviewing their learning about the answer to the questions.

Feedback

Give written feedback indicating the extent to which individuals have grasped the key ideas. Students with sound development may be introduced to quantitative aspects.

Commentary

Informal peer and self assessment is inherent throughout the group discussions (that promote listening, discussion and development of coherent ideas), the role play (that requires team working and cooperation), the demonstrations and the plenaries (when teacher questioning and review are essential).

The written summary gives teachers the opportunity to provide individual feedback based on the quality of the ideas they present.

Practical notes

Demonstration 1: hydrated barium hydroxide and ammonium chloride

This reaction is endothermic. The temperature drop is sufficient to cause water to freeze. The reaction vessel (glass beaker) can be made to ‘stick’ to a block of wood or heatproof mat.

Apparatus

• Eye protection
• Glass beaker, 250 cm³
• Thermometer reading -10–100 °C, or a temperature probe linked to a computer
• Heatproof mat or wood block
• Access to a fume cupboard

Chemicals

• Barium hydroxide-8-water (HARMFUL), 32 g
• Ammonium chloride (HARMFUL), 10 g
• Water

Procedure

1. Sprinkle a few drops of water on the mat or wood block.
2. Set the beaker on the water.
3. Mix the chemicals in the beaker, stirring with the thermometer or probe.
4. Keep the beaker on the mat.
5. As the temperature falls, the water will freeze so the beaker can be lifted with the mat underneath.

Demonstration 2: aluminium and iodine

This reaction is exothermic. The reaction will generate purple vapour, sparks and a flame. White aluminium iodide will remain in the dish.

Apparatus

• Eye protection
• Pestle and mortar
• Dish or metal tin lid
• Dropping pipette
• Heatproof mat
• Fume cupboard

Chemicals

• Iodine (HARMFUL), 2 g
• Aluminium powder (FLAMMABLE), 0.3 g
• Water

Procedure

1. Grind the iodine to a fine powder.
2. Mix the iodine and aluminium together in the mortar. Do not grind them together.
3. Set the dish on the mat in a fume cupboard.
4. Pile the mixture in the centre of the dish.
5. Gently add a few drops of water to the mixture.
6. Close the fume cupboard.
7. Wait a few seconds.

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection.
• It is the responsibility of the teacher to carry out appropriate risk assessments for the demonstrations.

Other equipment

• A multi-pack of crisps for the role play – the number of packs within the multipack can be varied according to the number of students in the class, although there is no need for everyone to be involved. A 12‑pack is the minimum recommended.

Answers

Will the reaction ‘go’?

Which reaction(s) would ‘go’ without any help?

• Only exothermic reactions will ‘go’ without any help.
  • False.
• To make an endothermic reaction happen, heat is needed.
  • False.
• Endothermic reactions don’t go because they don’t store energy.
  • False.
• Exothermic reactions store energy. This is released when they ‘go’.
  • False.
• It is impossible to predict if a reaction will ‘go’ or not until you try it.
  • False.
• Both reactions will ‘go’ but the endothermic reaction will be slower than the exothermic one.
  • True.

It’s all about chance

1. The chance of an event happening.
2. For example, not winning the lottery, a Premier League team winning the FA Cup, a non‑British player winning etc.
3. People setting the odds assess the likelihoods of the events – there is greater flexibility of outcomes in some events than others.
4. An athlete training hard, or in a specific way, or cheating; revising thoroughly; a government minister behaving in a way that influences electors etc.